1. Technical Field
The present invention relates to an exposure apparatus and an electronic device manufacturing method. More particularly, the present invention relates to an exposure apparatus including a field stop and to an electronic device manufacturing method performed by using the exposure apparatus. The present application is related to and claims priority from the following U.S. Provisional Patent Application, the contents of which are incorporated herein by reference.
2. Description of the Related Art
An exposure apparatus used for lithography irradiates, via an illumination optical system, a reticle with light generated by a light source, which is referred to as illumination light. The illumination light is transmitted through or reflected by the reticle. A substrate such as a silicon wafer is then irradiated, via a projection optical system, with the light transmitted through or reflected by the reticle, which is referred to as exposure light. In this manner, a photosensitive material applied to the substrate is exposed to light.
For exposure apparatuses, there is a demand for improved resolution as finer circuit patterns are desired. Therefore, the wavelength of the exposure light increasingly becomes shorter. The term “light” in the present specification refers to not only visible light but also electromagnetic waves having a wavelength shorter than 1 mm, in other words, including infrared rays and X rays. In recent years, exposure apparatuses using Extreme Ultraviolet (EUV) light having a wavelength of approximately 5 to 40 nm (hereinafter referred to as Extreme UltraViolet Lithography (EUVL) exposure apparatuses) have been suggested as next-generation apparatuses.
For such a very short wavelength range of EUV light, no existing materials have a sufficient transmittance to be used as refractive optical members. Therefore, a projection optical system used in an EUVL exposure apparatus comprises only reflective optical members. Since an EUVL exposure apparatus uses not a transmissive mask but a reflective mask, the illumination light needs to enter the mask (or reticle) at an angle for the following reason. When the illumination light vertically enters the reflective mask, the optical path of the illumination light that enters the mask overlaps the optical path of the illumination light that is reflected by the mask to be directed towards the projection optical system. Consequently, the optical members of the illumination optical system, which are configured to illuminate the mask, block the optical path of the projection optical system, or the optical members of the projection optical system block the optical path of the illumination optical system.
An EUVL exposure apparatus only provides a narrow and long arc-like effective exposure region (i.e., stationary exposure region) through a reflective projection optical system. For this reason, the mask pattern is transferred onto the photosensitive substrate in a scanning manner while the mask and the photosensitive substrate are moved relative to the projection optical system (a scanning exposure technique). Therefore, a field stop needs to be provided to define the stationary exposure region at a position that is substantially optically conjugated with the photosensitive substrate. In a conventional EUVL exposure apparatus, a field stop is disposed in the optical path of the illumination optical system, and an image-formation catoptric optical system is provided between the mask and the field stop. In this manner, the field stop is positioned so as to be optically conjugated with the mask (and consequently with the photosensitive substrate) (See U.S. Pat. No. 6,452,661).
Generally, the EUVL exposure apparatus exhibits a relatively low reflectance at each reflection surface. Therefore, in order to prevent loss of light and improve the throughput, it is desired to minimize as much as possible the number of reflections the light undergoes while traveling through the illumination and projection optical systems between the light source and the photosensitive substrate. Because of the image-formation catoptric optical system between the mask and the field stop as described above, however, the conventional EUVL exposure apparatus suffers from a relatively large number of reflections in the optical path of the illumination optical system. As a result, the conventional EUVL exposure apparatus experiences a large loss of light and thus cannot achieve the desired throughput.
To reduce the number of reflections in the optical path of the illumination optical system and thus achieve the desired throughput, the field stop may be positioned in the vicinity of the reflective mask (see Japanese Patent Application Publication No. 2005-108934).
When a foreign object is attached to the surface of a reticle of an exposure apparatus, all of the patterns transferred with the use of the reticle have a shadow of the foreign object. As a result, the product yield becomes significantly low. To prevent this problem from happening, an optically transparent film, which is referred to as a pellicle, may be provided so as to surround the reticle and catch foreign objects. Since the pellicle is distant from the surface of the reticle, no images of the foreign objects caught by the pellicle are present in the pattern image of the exposure light. If the pellicle is provided in the proximity of the surface of the reticle, however, it becomes difficult to position a field stop in the vicinity of the reticle.